Every year, a committee of experts sits down with a tough job to do: from among all ICREA publications, they must find a handful that stand out from all the others. This is indeed a challenge. The debates are sometimes heated and always difficult but, in the end, a shortlist of  the most outstanding publications of the year is produced. No prize is awarded, and the only additional acknowledge is the honour of being chosen and highlighted by ICREA. Each piece has something unique about it, whether it be a particularly elegant solution, the huge impact it has in the media or the sheer fascination it generates as a truly new idea. For whatever the reason, these are the best of the best and, as such, we are proud to share them here.


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  • Pulsations from a pulsar while accreting? Why not! (2021)

    Torres, Diego F. (CSIC - ICE)

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    Pulsations from a pulsar while accreting? Why not!

    In 1996, SAX J1808.4-3658 has been the first X-ray binary system discovered with an accreting millisecond pulsar, that is a neutron star spinning hundreds of times per second. Every 3-4 years it experiences outburst phases (9 since its discovery) during which its luminosity increases of several orders of magnitude and the system enters an accreting phase. Thanks to the mass and angular momentum transferred from the accretion disc, the pulsar is accelerated and rotates with a frequency of ~401 Hz (2.5ms period) generating pulsations in the X band. We now know 20 other binary systems like SAX J1808.

    Postdoctoral fellow Francesco Coti Zelati and ICREA Professor Diego Torres, both at the Institute of Space Sciences, were part of a team aiming to study the behavior of the pulsar while it was accreting. What they found was unexepcted: Using  SiFAP2, the Silicon Fast Astronomical Photometer and Polarimeter mounted at the Telescopio Nazionale Galileo astronomers were able to detect the pulsations of the ms-pulsar in the visible band. Towards the end of the burst they pulsations at UV wavelengths were also detected using the Hubble Space Telescope.

    This is surprising from a theoretical standpoint. Current accretion models fail to account for the luminosity in the visible and ultraviolet pulsed emissions, which are supposed to be driven from processes in the magnetosphere of the neutron star. It was widely believed that the charge density of accreting matter would shut off the acceleration of particles from the magnetosphere. The study suggests, instead, that acceleration of charged particles up to extremely high speeds can take place in the magnetosphere of a neutron star even when the latter is accreting matter.


  • How does the intestinal epithelium fold and move? (2021)

    Trepat, Xavier (IBEC)
    Batlle Gómez, Eduard (IRB Barcelona)

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    How does the intestinal epithelium fold and move?

    The human intestine is made up of more than 40 square meters of tissue, with a multitude of folds on its internal surface that resemble valleys and mountain peaks in order to increase the absorption of nutrients. The intestine also has the unique characteristic of being in a continuous state of self-renewal. This means that approximately every 5 days all the cells of its inner walls are renewed to guarantee correct intestinal function. Until now, scientists knew that this renewal could take place thanks to stem cells, which are protected in the so-called intestinal crypts, and which give rise to new differentiated cells. However, the process that leads to the concave shape of the crypts and the migration of new cells towards the intestinal peaks was unknown.  

    In this study we deciphered the mechanisms leading the crypts to adopt and maintain their concave shape, and how the migration movement of the cells towards the peaks occurs, without the intestine losing its characteristic folded shape. The study combined computer modelling, led by Marino Arroyo, professor at the UPC, with experiments with intestinal organoids from mouse cells, and shows that this process is possible thanks to the mechanical forces exerted by the cells.

    Using mouse stem cells and bioengineering and mechanobiology techniques, we developed mini-intestines, organoids that resemble the three-dimensional structure of peaks and valleys, recapitulating tissue functions in vivo. Using microscopy technologies developed by our group we carried out high-resolution experiments for the first time that allowed us to obtain 3D maps showing the forces exerted by each cell. 

    In addition, with this in vitro model, we showed that the movement of new cells to the peak is also controlled by mechanical forces exerted by the cells themselves, specifically by the cytoskeleton, a network of filaments that determines and maintains cell shape. 

  • Energy rebound under limited rationality, self-interest and willpower (2021)

    van den Bergh, Jeroen (UAB)

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    Energy rebound under limited rationality, self-interest and willpower

    The extent to which adopting energy-efficient technologies results in energy savings depends on how such technologies are used, and how monetary savings from energy efficiency are spent. Energy rebound occurs when potential energy savings are diminished due to post-adoption behaviour. It denotes that potential energy savings of adopting an energy-efficient technology or practice, possibly triggered by some policy, are offset by subsequent behavioural and systemic responses that increase energy use, resulting in diminished net energy savings.

    Three main types of rebound are: direct rebound or intensity-of-use effect – a technology becomes more energy-efficient and thus less costly in its use, causing consumers or producers to use it more intensively; indirect rebound or re-spending effect – spending less due to using a more energy-efficient technology releases money that is subsequently spent on other products or services that use energy over their life cycle; and economy-wide reboundmore energy efficiency leads to many other changes in the economy, such as investments in expansion of production, impacts on capital and labour markets, and indirectly increases in consumption, all with consequences for energy use. The figure illustrates these rebound types, and their joint impact on energy savings, for the case of switching to a more fuel-efficient car.

    We review empirical studies on how six behavioural regularities affect three energy-relevant decisions and ultimately rebound: adoption of energy-saving products or practices, their intensity of use and spending of associated monetary savings. The table with a summary of the findings indicates that behaviours that reflect limited rationality and willpower may increase rebound, while the effects of behaviours driven by bounded self-interest are less clear. We then describe how interventions associated with each of the behavioural regularities can influence rebound and thus serve to achieve higher energy savings. Future research ought to study energy-relevant decisions in a more integrated manner, with a particular focus on re-spending as this presents the greatest challenge for research and policy.

  • LOOK AT THE PHASE (2021)

    van Hulst, Niek F. (ICFO)

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    Concepts of holography, long associated to the world of 3D projections, entertainment and science fiction, are routinely applied in medical imaging, in fraud and security issues, in data storage, etc. Now holography is moving into microscopy, nanoscopy and bio-imaging applications. ICFO researchers Lisa Saemisch, Unai Ortiz-Orruño and Matz Liebel implemented holographic microscopy schemes to take advantage of the interferometric sensitivity and 3D-capacity, for ultrafast 3D particle tracking, particles sizing and particle identification.

    Merging ultrafast spectroscopy, wide-field nanoscopy, and digital holography they realized an ultrafast holographic transient microscope, enabling parallel 3D-tracking of fs-ps dynamics of many particles in wide field imaging (Figure 1).

    By imaging in the Fourier plane (k-space) the scattering signal of all particles was projected onto all camera pixels, thus dramatically boosting the achievable dynamic range for quantitative sizing (Figure 2).

    Finally, directly visualizing the phase response of individual nanoparticles they devised a simple wide-field method for distinguishing metallic and dielectric particles purely based on their spectral phase behavior.

    These results pave the way to single-shot 3D microscopy of 2D and 3D materials on arbitrary time scales from femtosecond carrier dynamics in advanced materials to millisecond dynamics in complex tissues and cells.

  • What is language for? (2021)

    Wiltschko, Martina E. (UPF)

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    What is language for?

    The capacity for language sets humans apart from all other animals. As long as scholars have been thinking about language there has been a debate as to why it evolved: is its core function to configure human-specific thought or is it rather mainly a tool for communication? The debates surrounding this question typically evolve around aspects of language that are used to express thoughts like "The dog is sleeping in my bed." However, there are other aspects of language which are dedicated to regulate communication and the expression of thought in conversational interaction is typically enriched in this way. Interactional aspects of language are used to regulate the construction of common ground as well as the interaction itself (e.g., turn-taking). Without interactional language (eh, yeah, right) the below conversation would sound curt, wheras its addition adds an interactive component that conveys that the speakers care about understanding each other.

    I:  Gal Gadot was amazing as Wonderwoman, eh? R: Yeah, I know, right?

    The fact that the language dedicated to interaction is as much part of language as the language dedicated for the expression of thought suggests that the function of language cannot be reduced to one or the other. And in fact, the same logic that configures the language of thought also configures the language of interaction. This logic, which we may refer to as a grammar of sorts, is arguably part of our genetic endowment, as evidenced by the fact that it is human-specific (animals communicate but they have no means to regulate their communication) and its acquisition is automatic (children automatically aquire interactional language just as they do the language they need to express the thoughts they want to communicate). In this way, the distinction between thought and communication dissolves: language unites them.

  • Neandertal Art (2021)

    Zilhão, João (UB)

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    Neandertal Art

    Analysis of the splattered red pigment covering a large stalagmite dome in the Sala de las Estrellas of Ardales cave (Malaga, Spain) proved their anthropogenic nature, showed the mineral sources used must have been located outside the cave itself, and revealed variations in composition that correspond to the different ages as previously measured by U-Th dating, thereby corroborating the Neanderthals' symbolic used of this underground space over an extended time span beginning >65,000 years ago.